1. Technical Field
The present invention relates to a method and apparatus for monitoring wheel conditions on a moving truck and more particularly to a method and apparatus for detecting wheel alignment and inflation problems and for monitoring load positioning.
2. Background to the Invention
Poor wheel alignment and tire under inflation are factors contributing to premature tire wear and poor vehicle handling. A system which alerts a vehicle operator of low tire pressure and misalignment would help control one of the factors contributing to the accelerated wear of tires. Unfortunately, the direct provision of pressure sensors in wheels to monitor tire pressure has proven unreliable and expensive. Pressure sensors mounted in wheels are subjected to a hostile environment of high temperatures, road shock and high rotational velocities. The data transmission linkage from the wheel to the vehicle is necessarily indirect and adds expense to the system, and it requires an independent power supply, such as a battery, which must periodically be removed from the tire for replacement.
Tire pressure directly relates to the rolling radius of a tire, which shrinks with reduced pressure. As the tire's rolling radius is compressed, side wall flex increases causing the tire's temperature to increase. Increased operating temperature of a tire promotes degradition of the synthetic rubber of the tire and results in excessive tire wear. As an alternative to measuring tire pressure, a wheel's rotational speed can also be measured as an indirect indication of tire under inflation. The increasingly widespread use of antilock braking systems (ABS) on cars and trucks affords a convenient source of wheel rotational speed information which can be used to implement pressure monitoring methods involving wheel rotational speed monitoring.
U.S. Pat. No. 6,064,936 to Nakajima describes a system utilizing an ABS to provide inputs for tire pressure monitoring system. While increases in one wheel's velocity suggest low inflation of the tire mounted on that wheel, other circumstances may exist which be the cause of a relative increase in wheel angular velocity. The Liu, et al. reference, U.S. Pat. No. 5,760,682 develops calibration coefficients to compensate for factors such as wheel slippage, rough road fluctuations, vehicle cornering, and uphill and downhill travel. Desirable though is a system which can identify different conditions affecting wheel rotational velocity so that the proper corrective steps may be taken.
Numerous patents relate specifically to the application of indirect tire pressure measurement using ABS wheel speed sensors on automobiles. With trucks additional issues are raised due to the much higher sidewall stiffness of tires compared to automobile tires (and consequent relatively lower sensitivity of rolling radius to tire pressure) and the differences in wheel layout used for trucks, for example 4.times.6 (a front steering axle with two tires with a rear driven axle carrying four tires) and 6.times.10 (a front steering axle with two rear driven axles, each with four tires) wheel arrangements. Truck suspensions are designed for maximum load conditions and loading varies much more than for passenger autos. Vehicle speed for trucks is commonly measured by a tachometer. The tach is attached to track the rotational velocity of a drive shaft installed between the vehicle's transmission and a rear end differential. As a result measured velocity is proportional to the average rotational velocity of the driven wheels. ABS wheel sensors provide an alternative source of data with which to generate vehicle speed, among values for other operating variables.